There are plenty of vulnerabilities to exploit if his opponents can summon the will and the muscle.

Even as he won in South Carolina, Mr. Trump seemed to overstay his welcome, revealing a tendency to test voters’ patience. In South Carolina, late-deciding voters made up 45 percent of the Republican electorate, and they uniformly scorned Mr. Trump. And surveys suggest that even some Republicans find him unlikable and lacking in compassion.

What’s more, so much of Mr. Trump’s campaign and his conduct remain startlingly unpredictable, from his spats with the pope to his shifting memories of his previous positions on momentous issues, such as his opposition (then later support, then opposition again) for the American-led invasion of Iraq.

And Mr. Trump will soon lose a feature of the 2016 race that has disproportionately benefited his candidacy: a large and unwieldy field of rivals that carved up much of the Republican electorate into small slivers.